class Solution
{
    vector<int> tmp;
public:

    int reversePairs(vector<int>& nums)
    {
        tmp.resize(50001);
        return mergesort(nums, 0, nums.size() - 1);
    }

    int mergesort(vector<int>& nums, int left, int right)
    {
        if (left >= right)
        {
            return 0;
        }

        int ret = 0, mid = (left + right) / 2;

        ret += mergesort(nums, left, mid);
        ret += mergesort(nums, mid + 1, right);

        int cur1 = left, cur2 = mid + 1, i = 0;
        while (cur1 <= mid && cur2 <= right)
        {
            if (nums[cur1] <= nums[cur2])
            {
                tmp[i++] = nums[cur1++];
            }
            else
            {
                ret += mid - cur1 + 1;
                tmp[i++] = nums[cur2++];
            }
        }

        while (cur1 <= mid)
        {
            tmp[i++] = nums[cur1++];
        }
        while (cur2 <= right)
        {
            tmp[i++] = nums[cur2++];
        }

        for (int i = left;i <= right;i++)
        {
            nums[i] = tmp[i - left];
        }

        return ret;
    }
};
